Earthmoving vehicles, such as wheel and track dozers and loaders, are frequently employed in grading, clearing, and snow removal operations where it is desirable that the ground engaging implement or work tool follow the contour of the supporting surface. For example, when such a vehicle is used to remove snow from a paved surface, it is undesirable to have the implement dig into the pavement and impart damage thereto. To this end, such vehicles are equipped with an implement hydraulic circuit, which, when placed in the so called "float" mode, permits the work tool to rest on the supporting surface by virtue of the force of gravity acting thereon and thereby follow the supporting surface contour.
Hydraulic systems employed in this type of vehicle to control the raising and lowering of the work tool commonly include a pilot operated control valve having a control spool shiftable in response to a shifting force resulting from selectively directing pilot pressure thereto. The work tool is controlled by shifting the spool in the control valve to either a power up or raise position, a neutral or hold position, or a power down or lower position. The float position is commonly obtained by shifting the control circuit spool to a fourth position beyond power down. This type of float requires additional spool travel in the lower direction over that required in the raised direction. In a pilot operated system this additional spool travel requires a considerable increase in shifting force above what is required to attain the power up or power down positions. Thus, a proportional amount of pilot supply pressure must be reserved for this additional shift, and the pilot pressure range alloted for raise and lower modulation is reduced.
Prior art pilot operated control systems employ pilot control pumps and control systems that provide added pilot pressure to obtain additional spool travel in response to shifting of an infinitely variable pilot valve. In systems of this type, shifting of the pilot valve results in a metering effect, in that the pilot pressure delivered to the control spool varies in a relationship directly proportional to the displacement of the pilot control valve, thereby providing a higher pilot supply pressure than the normal shift pressure to achieve the additional spool travel.
The foregoing illustrates limitations of the known prior art. In view of the above, it would be advantageous to provide a control system for a pilot operated valve which would permit the additional spool travel required to obtain a float position and yet preserve a large percentage of pilot source pressure for the raise and lower modulation positions.